What you want to do is create an image of the SD card. The exact details depend on your computer operating system. You fire up some software, point it at the card, and make an image which you store on your computer. Then you can write that image back to a card later.

You may already have such software on your computer (e.g. some Nero products often come preinstalled), or for some operating systems you can use the command line. This is a Windows one that they mention at XDA.http://www.roadkil.net/program.php?ProgramID=12

Just be careful at the time you write the image back. If you designated the wrong drive to write to, obviously it would be a big disaster.